1. Field of the Invention
This invention pertains to large diameter fluid flow ducts for use in ocean engineering and the like. More particularly, it pertains to a connection of the upper end of the pipe to a floating structure, which connection is so arranged that motions of the floating structure, such as pitch and roll motions, are not transmitted to the duct where they would induce bending of the duct.
2. Review of the Prior Art and Its Problems
Many sophisticated proposals have been made in the field of ocean engineering which call for the use of large diameter vertical ducts of great length extending from at or near the ocean surface to lower ends unconnected to the ocean floor. These proposals include concepts for ocean thermal energy conversion and for mariculture.
The ocean thermal energy conversion (OTEC) concepts propose to use the difference in thermal energy levels between warm surface water and colder deep water to generate electricity, for example. The available energy level difference is low and so these proposals rely on the use of very large quantities of warm and cold water, and call for the necessary large volumes of deep ocean cold water to be brought to the water surface through very large vertical ducts of great length. The rate of water flow through the ducts, called "upwelling ducts", would be low, and thus the pressure differential across the walls of the duct would be correspondingly low, especially if the water in the duct is not carried far above the ocean surface before being discharged from the duct.
Any structure which extends vertically for any significant distance in the ocean will encounter at least one ocean current. Currents impose drag forces upon such structure. The larger the structure, the greater its profile (effective area) presented to the current, and therefore the greater the drag forces which a given current will impose on the structure. These drag forces impose bending loads on the structure, and the longer the structure, the more severe the resulting bending stresses. The problem of ocean current drag forces and of the bending stresses produced thereby is a significant difficulty in the offshore drilling industry in regard to the riser pipes which are used to conduct drilling mud during the drilling of a subsea oil or gas well from the well bore to the surface drilling platform. Current-induced bending stresses are so great in these riser pipes that the pipes must be made very heavy, i.e., with thick walls, to enable them to withstand the bending stresses. This problem is such that, because of it, offshore drilling operations today are effectively limited to water depths of about 1000 feet or so. This limit is imposed principally by the riser pipes.
In commonly-owned copending patent application Ser. No. 886,904 filed Mar. 15, 1978, there is described on OTEC upwelling pipe composed of individual lengths of synthetic pipe interconnected by flexible couplings. The pipe assembly is disposed about an elongate tensile core element which carries the weight of the entire pipe assembly. As described in commonly-owned copending application Ser. No. 886,907 filed Mar. 15, 1978, the tensile core of the pipe assembly can also carry the weight of a stabilizing mass suspended below the lower open end of the upwelling duct as such. These arrangements meaningfully address the problem of bending stresses induced in an upwelling pipe due to ocean current drag forces.
An OTEC facility would necessarily have to be located in very great depths of water in order that the upwelling pipe can extending sufficiently far below the water surface to reach into waters having a usefully great temperature difference from the warmer surface water. In such instances, the heat exchange and power generating equipment would necessarily be located upon a floating structure at the surface of the water. Any floating structure would be subjected to rolling and pitching motions by wave actions upon the structure. The upper end of the upwelling duct would be connected to the floating structure. Angular motions of the floating structure would be transmitted to the upwelling duct in such manner as to create substantial bending moments in the upwelling duct if the connection of the upper end of the duct to the floating structure were a rigid connection, regardless of the manner in which the upwelling duct itself is defined. Whether the upwelling duct is defined according to the descriptions in the above-identified commonly-owned copending applications or not, the connection of the upwelling duct to the floating structure must be capable of accommodating substantial vertical loads.
A need is therefore seen to exist for a connection arrangement useful between the upper end of an OTEC upwelling duct and a floating OTEC facility, for example, which is capable of accommodating substantial vertical loads occurring across this interface, and yet which does not transmit motions of the floating structure to the duct. The isolation of angular motions of the floating structure from the upwelling duct is desirable to eliminate the creation in the upwelling duct of bending moments. The greater the extent to which bending moments can be avoided in the upwelling duct, the more straightforward the structure of the duct can be, and the lower its cost.